Foamed sheets of polyester are often made by extruding a polyester composition out of an annular die and over a "cooling can" after which the somewhat cooled polyester is cut to form a sheet that is taken up on a roll. A common problem encountered in extruding molten polyester onto a moving belt is that the polyester extrudate tears as it is being pulled over the cooling can. This type of failure is often due to the lack of adequate polyester polymer melt strength. A polyester polymer possessing high melt strength resists the tendency of tearing or ripping when subjected to tensile forces. Accordingly, it would be desirable to increase the melt strength of an extrudable polyester polymer to allow a continuous take up of foamed sheet onto a roll and reduce the down time needed to re-align a torn extrudate onto or over a cooling can.
U.S. Pat. No. 4,544,677 is directed to foamable thermoplastic compositions made with a mixture of polycarbonate resins, optionally polyester resins, a foaming agent, and a nucleating agent having a specific aspect ratio. The optional polyester resins disclosed are linear, and additional amounts of acid or glycols could be added in amounts ranging from 0.5 wt. % to 50 wt. % based on the total composition. We have found, however, that linear polyesters are not well suited to extruding foamed sheets because the resin composition has insufficient melt strength. Without sufficient melt strength and die swell, the molten resin both rips and tears as it is pulled over or onto the cooling can, and fails to yield a product with low density. Certain foaming methods, such as injection molding techniques as described in U.S. Pat. No. 544,677, do not require the use of a resin of great melt strength because the resin is not placed under tension or taken up during the foaming of the resin. Accordingly, there remains a need to develop a resin which has high melt strength.
U.S. Pat. No. 5,360,829 discloses foamed polyester sheets made from amorphous polyesters, 20 to 100 parts by weight of a polycarbonate resin based on 100 parts by weight of the polyester, and a nucleating agent in order to impart to the foamed sheet a uniform cellular morphology and a high foaming ratio while maintaining its mechanical properties and dimensional stability. The patentees noted that when crystalline polyesters were blended with polyolefins, the cellular morphology of the resulting foamed sheets was not uniform, and the foaming ratio was too low. Accordingly, the patentees recommended using amorphous polyesters in combination with polycarbonates to overcome this deficiency. Amorphous polyesters, however, even in foamed compositions, suffer the drawback in that they are excluded from being used in high temperature applications. It would be desirable to find an alternative polyester composition that possesses high melt strength without this noted deficiency.
The foamed sheets of polyester find a number of consumer uses, many of which subject the polyester foam sheet to high impacts. Therefore, it is also desirable to make a polyester foam sheet which has high impact strength. One way of doing so is to increase the density of the polyester foam, either by increasing the amount of polymer per unit volume, or by adding reinforcing agents. Neither method is an attractive solution. Both increase the cost of the foam. Further, in the latter method, the amount of reinforcing agent that can be added is limited by the increase in viscosity to the polymer melt resulting from the reinforcing agent.
Attempts to decrease the density of the polyester foam have also lead to a corresponding decrease in the impact strength of polyester foams. It would be highly desirable to make a polyester polymer that overcomes this longstanding direct relationship between the foam density and impact strength; that is, there exists a need to make a polyester foam that simultaneously has a low density and an improved impact strength. Accordingly, in addition to making a polyester polymer which processes well by virtue of its high melt strength, as an additional embodiment, it is desirable to make a polyester foam sheet, and a thermoformed article from the sheet, which has a good cellular structure, low density, and high impact strength.
U.S. Pat. No. 5,502,119 suggests stabilizing blends of polyesters and aromatic polycarbonates with a particular organosilicate. The types of polyesters described therein are not polycondensed branched types of polyesters. U.S. Pat. No. 5,504,130 describes filled thermoplastic compositions where about 15-50% of at least one poly(phenylene ether) resin is compatibilized with about 20-80% of at least one polyester resin by using from 3-50%, preferably of from about 8-20%, of an aromatic polycarbonate having a weight average molecular weight of at least about 40,000. These patents, however, do not address the problem of how to improve the melt strength of polyester compositions, or how to additionally make foamed sheets having good cellular uniformity and high impact strength at low density.